i am not sure if this is an option available in scrivener mac. i am a windows user and have been waiting for this beta for quite some time. (forgot this was becoming available for nanowrimo 2010) i use the thesarus alot when i’m writing and it is very convenient not to carry around a large hardcopy when i’m traveling. 
I don’t think there are any current plans to provide a thesaurus or dictionary (other than the spelling dictionary, which doesn’t have definitions) with Scrivener for Windows. In Mac OS land, Apple provides a dictionary/thesaurus/wikipedia/spell check tool that all applications can hook into, so that stuff isn’t part of the Scrivener code base.
If you really want a good thesaurus on the go, even when you are offline, I think you’ll want to start googling for software to fill that gap for you.
Try wordweb it’s excellent, and both offline and online. They have both a free and pay version and I use it far more than I should. 